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Nitrogen (N) fertilization has been repeatedly reported to strongly influence soil properties and crop growth. However, there is little information about the combined effects of soils and crops on soil CO2 fluxes under N fertilization. In this study, in situ soil respiration, soil physicochemical properties, microbial communities and crop properties were measured for eight years (2008 to 2016) on the Loess Plateau. Five rates of N fertilization were applied to different plots to compare the soil respiration rate and its temperature sensitivity (Q10). Nitrogen fertilization significantly increased mean annual cumulative soil respiration (Rcum) by 25%–44%. Rcum had a positive correlation with grain yield, and the carbon emission efficiency (grain yield produced per unit of carbon emission) under N-fertilized plots was 1.62–2.52 times that of unfertilized plots. Rcum also had a positive correlation with root biomass and the root N concentration but showed a negative correlation with the root C/N ratio. The Q10 values under N-fertilized plots decreased by half at a diurnal scale, but had a smaller reduction (i.e., 0.04–0.09) at an annual scale compared to unfertilized plots. The decreased Q10 values under N-fertilized plots also resulted from the lower aromaticity of dissolved carbon (SUVA254) (7.40 vs. 10.53 L mg C−1 m−1). In addition, the altered Rcum and Q10 values were affected by the varied bacteria community derived from N fertilization, which was related to Acidobacteria, Chloroflexi, Proteobacteria and Bacteroidetes. Therefore, N fertilizer applications regulate the combined effects of soil and crop parameters on soil respiration and the Q10 value. This study suggests that, due to the lower carbon emission efficiency and higher SOC concentration under N-fertilized plots, N fertilizer applications may be used to sustain crop yields and increasing SOC storage while minimizing carbon emission impacts to the environment on the Loess Plateau.

Soil erosion influences both lateral soil organic carbon (SOC) re-distribution and vertical soil CO2 emissions. While potential SOC mineralization during transport and the burial effects of SOC at depositional sites have been addressed in previous reports, erosion induced on-site CO2 emissions are still under-studied. In this study, two soils (Loess soil and Black soil) with similar texture but contrasting aggregate structure and SOC content were subject to a set of 60-min long simulated rainfall events. There were two different rainfall intensities (30 and 90 mm h−1) at three slope gradients (5°, 15° and 25°). Runoff and sediment from erosion plot were collected at 10-min intervals over 60 min. Soil CO2 emissions from eroding slopes, SOC and particle size distribution of the eroding soil were measured after the erosion events. The results show that the runoff rates from the two soils were comparable, but the sediment rates from the Loess soil roughly three times that from the Black soil. In general, the SOC erosion from the Loess soil was 1.8 times that from the Black soil, even though the SOC concentration in the original Black soil was 56% higher than the Loess soil. The cumulative soil CO2 emissions from the eroding slopes of the Loess soil ranged from 15.4 to 19.7 g C m−2, which was doubled on the Black soil (from 28.1 to 59.6 g C m−2). When the rainfall intensity raised from 30 mm h−1 to 90 mm h−1, the cumulative soil CO2 emissions from the Black soil decreased by 38.2%, but only declined by 10.0% on the Loess soil. When the slope gradient increased from 5° to 25°, the cumulative soil CO2 emissions decreased by 23.8% on the Black soil but by 12.6% on the Loess soil. Therefore, our observations suggest that the soil CO2 emissions on the Black soil was much more sensitive to the variations of rainfall intensity and slope gradients than the Loess soil. Greater SOC erosion should not be directly translated to less on-site soil CO2 emissions. The selective depletion/enrichment of SOC and the lability of individual components must be fully understood when accounting for slope-scale carbon balances.

Two-dimensional (2-D) numerical models have been widely used to predict soil water-salt transport under mulched drip irrigation. However, conventional 2-D models often neglect the spatial variation of soil water-salt transport along drip lines and over-simplifies it by averaging values. This calls for a more robust model to better reflect the actual spatial and temporal variation of soil water-salt distribution in the field, especially in arid regions using brackish water with additional salt. In this study, mulched drip irrigation with brackish water was applied to a cotton field with loam soil in an arid region of southern Xinjiang, northwest China. The changes of soil water potential and total dissolved solid (TDS) of soil water over two irrigation events were intensively measured on hourly base to establish and calibrate a 3-D model, and another five irrigation cycles were monitored on daily base to validate the 3-D model. The mean absolute relative errors between measured and calculated soil water potential and TDS of soil water were 13.7% and 10.7% during hourly-based model calibration, even though there might be bias inevitably introduced by pre-determined sampling intervals and volumes. The calculated values during model validation were reasonably in line with the temporal patterns of soil moisture and TDS before and after irrigation at different irrigation cycles. This 3-D model was then applied to predict the spatial and temporal variations of soil water-salt transport during and after irrigation. Our results show that (1) the 3-D model with additional consideration on point-source discharge from individual drippers effectively reflected the wet front interferences along the drip lines. (2) A semi-elliptic cylindrical wet bulb together with relatively low salinity was formed along the drip lines, which matched well with the cotton layout (one mulch, two drip lines and four rows). However, the uneven overlapping of wet front interferences among individual drippers required to optimize the design of dripper intervals and irrigation regime to provide desirable soil water-salt conditions for cotton growth in this loam soil. (3) During the 96 h after irrigation, the TDS of soil water was always greatest in the dense root zone, increasing from approximately 3.8 g L−1 to 7.0 g L−1 as a result of root water uptake. Such localized re-salinization patterns demand repeated leaching in continued irrigation cycles to ensure cotton growth. Our study suggests that a smaller interspace between drippers with short but more frequent irrigation cycles would be much more helpful to timely and spatial-precisely meet plant water demanding.

Spatial heterogeneity of soil microbes introduces great uncertainty to our understanding of microbe-mediated soil carbon cycling, yet was few studied on sloping lands. Along a steep-slope grassland (35°) on the Chinese Loess Plateau, soils of 0–10 cm were sampled in 2016 at three slope positions (upper, middle and bottom) to determine microbial community composition (by Illumina Hiseq sequencing) and function (enzymes involved in carbon cycling, the in situ soil respiration and temperature sensitivity). The bacterial alpha-diversity were greater at middle- and bottom- than at upper slope position, while fungal alpha-diversity varied little across slope positions. The bacterial phylum Proteobacteria was 9.7% and 19.4% lower but Acidobacteria was 36.5% and 41.3% greater at bottom- than at upper- and middle- slope positions. The fungal community transitioned from being Basidiomycota-dominant (relative abundance of 46.8%) at upper slope position to Zygomycota-dominant (relative abundance of 36.5%) at bottom slope position. The β- d -glucosidase activity generally declined down the slope while β- d -xylosidase and cellobiohydrolase activities hiked at middle slope position. All the enzyme activities were suppressed at bottom slope position. Soil respiration increased by 49.1% (P

Temperature sensitivity of soil respiration (Q10) is an important parameter when modeling the effects of global warming on terrestrial ecosystem carbon release. Widely applied chemical fertilizers can significantly affect soil productivity and carbon cycling in agroecosystems. However, little is known about how Q10 responds to chemical fertilization under different levels of initial soil fertility. On the Chinese Loess Plateau, changes in soil respiration rates and Q10 were investigated in soils of two fertility levels: low fertility (L) and high fertility (H). For each soil fertility level, there was one control plot and one chemical fertilized plot (+NP), which in total formed four treatments: L, L + NP, H and H + NP. All the treatments were replicated for three times on a continuous winter wheat cropping system. Respiration rates of surface soil in each treatment were in situ monitored from October 2010 through September 2015. Our results showed that after NP fertilization, soil respiration rates were increased by 46% in low fertility soil, yet only by 14% in high fertility soil (P

Land use plays an essential role in regional carbon cycling, potentially influencing the exchange rates of CO2 flux between soil and the atmosphere in terrestrial ecosystems. Temperature sensitivity of soil respiration (Q10), as an efficient parameter to reflect the possible feedback between the global carbon cycle and climate change, has been extensively studied. However, very few reports have assessed the difference in temperature sensitivity of soil respiration under different land use types. In this study, a three-year field experiment was conducted in cropland (winter wheat, Triticum aestivum L.) and apple orchard (Malus domestica Borkh) on the semi-arid Loess Plateau from 2011 to 2013. Soil respiration (measured using Li-Cor 8100), bacterial community structure (represented by 16S rRNA), soil enzyme activities, and soil physicochemical properties of surface soil were monitored. The average annual soil respiration rate in the apple orchard was 12% greater than that in the cropland (2.01 vs. 1.80μmolm-2s-1), despite that the average Q10 values in the apple orchard was 15% lower than that in the cropland (ranging from 1.63 to 1.41). As to the differences among predominant phyla, Proteobacteria was 26% higher in the apple orchard than that in the cropland, whereas Actinobacteria and Acidobacteria were 18% and 36% lower in the apple orchard. The β-glucosidase and cellobiohydrolase activity were 15% (44.92 vs. 39.09nmolh-1g-1) and 22% greater (21.39 vs. 17.50nmolh-1g-1) in the apple orchard than that in the cropland. Compared to the cropland, the lower Q10 values in the apple orchard resulted from the variations of bacterial community structure and β-glucosidase and cellobiohydrolase activity. In addition, the lower C: N ratios in the apple orchard (6.50 vs. 8.40) possibly also contributed to its lower Q10 values. Our findings call for further studies to include the varying effects of land use types into consideration when applying Q10 values to predict the potential CO2 efflux feedbacks between terrestrial ecosystems and future climate scenarios.

The influence of vegetation coverage and topography on soil organic carbon (SOC) stocks has been intensively studies. However, very few of the studies have recognized the potential combining effects of vegetation types and topographic positions onto SOC distribution, especially on the Chinese Loess Plateau where vegetation recovery has generated complex combination of fragmented topography and vegetation coverage. This study systematically sampled soil cores (259) from four vegetation types (woodland, grassland, cropland, and orchard) at three topographic positions (tableland, slope and valley bottom). Each soil core was divided into three layers: surface soil (0–20 cm), subsoil (20–60 cm) and deep soil (60–200 cm). Our results show that: (1) the SOC concentration declined over soil depths, regardless topographic positions or vegetation types. The absence of ancient cultivation layers at the valley bottoms further made the SOC stocks deep to 200 cm there much less than the tableland with thick loess soil layers (8.3 kg km –2 vs. 13.4 kg km –2 ). (2) The SOC concentration of cropland varied evidently with topographic positions, with the greatest on the tableland (8.0 g kg –1 ), and the least along the slope (5.3 g kg –1 ). However, grassland was rather stable across the three topographic positions. (3) In addition, the SOC concentrations of the three vegetation types were comparable on the tableland (6.1 g kg –1 ), while differed noticeably at the valley bottoms (5.0 g kg –1 ). Overall, our findings in this study call for the account for each combination of topographic position and vegetation type, so as to properly assess regional SOC stocks for sustainable land use.

Knowledge of CO2 emissions under different slope gradients and positions and its controlling factors is critical in accurately estimating CO2 emissions and carbon cycling on the slopes of eroded regions. In this study, three east-facing plots of 100 m2 (20 m × 5 m) with a slope gradient of 0.5° (S0.5), 1° (S1), and 3° (S3) were established in an eroded gully of the semi-arid Loess Plateau, China. The CO2 emission, temperature, moisture, runoff, sediment, fine root biomass and grain yield of these three plots were measured from October 2013 to September 2015 to investigate the relationship between slope gradients and soil CO2 emissions. The results showed that the mean annual cumulative CO2 emissions at S1 and S3 (731.0 ± 65.1 and 628.3 ± 74.8 g C m−2 year−1) were about 13.4% and 25.5% lower than that at S0.5 (843.7 ± 84.9 g C m−2 year−1). The CO2 emissions were higher at bottom slope than at upper slope, with an increase of 26.2% at S3, 22.9% at S1 and 14.5% at S0.5, respectively. The mean soil moisture ranged from 40.8% to 44.8% water-filled pore space (WFPS) among the slope gradients, and from 35.8% to 45.6% WFPS among the slope positions. There was a significant difference in mean fine root biomass among different slope gradients (S0.5 > S1 > S3, P

Soil C-acquiring enzymes are good indicators for the biological mechanism of soil nutrients and organic matter cycles. However, they have been used less frequently to assess the ecological stability and soil C cycle in eroding landscapes due to a lack of knowledge of the responses of C-acquiring enzyme activities to soil erosion and deposition. In the present study, a 3-year field simulation experiment was conducted to examine the variations in the activities of C-acquiring enzymes (β-1,4-xylosidase (βX), β-1,4-glucosidase (βG) and β-D-cellobiohydrolase (CBH)) from erosion-deposition plots with different slope gradients (5°, 10° and 20°) on the Loess Plateau in China (2016–2018). The activities of βX, βG and CBH were higher in the depositional plots than in the erosional plots, and those differences were enlarged with increasing slope gradients. Compared to the 5°-erosional plot, the activities of βX, βG and CBH respectively declined by 3.2–4.5%, 14.3–37.5% and 12.7–29.1% in the 10°-and 20°-erosional plots. The βX, βG and CBH activities were 2.2–18.1%, 17.3–32.1% and 14.8–86.2% higher in the 10°- and 20°-depositional plots than in the 5°-depositional plot. Moreover, the total soil CO2 emissions from the whole erosion-deposition plots decreased as slopes steepened. The displaced runoff and sediment depleted soil moisture, SOC, clay and microbial biomass in the erosional plots but enhanced these resources in the depositional plots, which can account for the changes in C-acquiring enzyme activities. The spatial distribution of enzyme activities affected soil CO2 emissions in a positive linear function. The sensitive responses of the C-acquiring enzyme activities and the controlling effects of C-acquiring enzyme activities on soil CO2 emissions during erosion and deposition processes, should be properly considered in assessing the biological mechanism for nutrition cycling in regions predominated with fragmented eroding landscapes.

Soil prokaryotes composition and diversity are the key to uncovering the mechanisms that drive variations in soil biogeochemical processes. Soil erosion is a primary factor that affects the spatial distribution of the soil prokaryotic community, but how soil prokaryotes in soil-eroding catena respond to environmental factors related to topography remains largely unclear. In this study, topsoils were sampled from three typical erosion geomorphic units (autonomous, transitional and depositional zones) in 2018 to identify the soil prokaryotic community and interactions among the species on the Chinese Loess Plateau. The alpha-diversity was greater but the beta-diversity was lower in the autonomous and transitional zones than in the depositional zone. Gammaproteobacteria and Bacteroidetes were 73% and 68% lower in the autonomous and transitional zones than in the depositional zone. In addition, Deltaproteobacteria, Acidobacteria, Firmicutes, Actinobacteria, Chloroflexi, Gemmatimonadetes and Nitrospirae were significantly higher in the autonomous and transitional zones. A less clustered network and fewer co-occurrences within the prokaryotic community and functional groups of processes were found in the depositional zone than in the autonomous and transitional zones. The alpha-diversity index was significantly negatively correlated with clay particles, soil water content, soil organic carbon (SOC), and ratio of SOC and nitrogen (C/N) but positively correlated with total nitrogen (TN). The higher relative abundances of copiotrophic groups (including Gammaproteobacteria, Bacteroidetes, etc.) in the depositional zone was mainly due to the increased SOC caused by the deposition of SOC-rich clay. Reassembly of the soil physico-chemistry characteristics among the topographic units significantly altered the soil prokaryotic community along the soil-eroding catena.

During slope-scale erosion events, re-distribution of eroded soil and the associated soil organic carbon (SOC) is not always uniform, but very often affected by preferential transport and deposition. Under given flow conditions, the site of SOC deposition depends on the transport distances of sediment particles containing the SOC. Very often, soil and SOC erosion risk are assessed by applying mineral particle specific SOC distributions to erosion models. However, soil is not always eroded as individual mineral particles, but mostly in a form of aggregates. Aggregates are likely to increase settling velocities of individual mineral particles, which may considerably reduce the transport distance of sediment fractions and the associated SOC, skewing SOC redistribution and its subsequent fate. Yet, little is known about the potential effects of aggregation on the movement and fate of eroded SOC. To assess the effect of preferential deposition, a simulated rainfall was applied to two soils in this study, with the Movelier silty clay having greater SOC content and aggregate stability than the Mohlin silty loam. The eroded sediments of the two soils were fractionated by a settling tube apparatus according to their potential transport distances. The CO2 emissions of the fractionated and incubated sediments were then measured for 50 days enabling the assessment of the bioactivity of eroded SOC for weeks after deposition. Our results show that: 1) the re-deposition of eroded SOC into terrestrial systems increased by 64% if considering the actual aggregate specific rather than the mineral particle specific SOC distribution. 2) The CO2 emission rates differed across settling fractions, with the most pronounced rates in the finest fractions from the Mohlin silty loam sediment and in the medium-size fractions from the Movelier silty clay sediment. 3) Over 50-day incubation, the CO2 emissions from the Mohlin silty loam sediment was 114% greater than that from the non-eroded Mohlin soil, whereas CO2 emissions were roughly equivalent for the Movelier sediment and non-eroded Movelier silty clay. These data demonstrate that erosion and preferential deposition of SOC-enriched aggregates can enhance terrestrial SOC deposition. This can further result in greater CO2 efflux than commonly applied mineral particle size specific SOC distribution would suggest. The different performances of deposition and CO2 emissions between the two soils also suggest that these effects can vary with soil type. Our observations certainly illustrate that sediment aggregation requires further investigation to assess the redistribution and subsequent fate of eroded SOC appropriately.

Rocks in civil and mining engineering usually experience rather complex stress disturbance. Rock dynamic mechanical behaviors under constant stress amplitude condition have been widely studied. However, the rock fracture evolution characteristics subjected to multi-level cyclic loading conditions are not well understood. In this work, multi-level cyclic compressive loading experiments were performed on marble with interbed orientation of 0°, 30°, 60° and 90°. Anisotropic fracture evolution characteristics were revealed using dynamic stress strain descriptions and post-test CT scanning technique. Results show that rock fatigue deformation, strength, lifetime, dynamic elastic modulus and damping ratio are all impacted by rock structure. The interbed structure plays a dominant role in fracture evolution compared with natural fracture and pyrite band. Rock stiffness degrades and damping ratio increases with the increase of interbed orientation. In addition, a two-phase damage accumulation pattern was found for marble under multi-level cyclic loading condition. A damage evolution model was first established to model damage accumulation, the proposed model fits well to the experimental data. Moreover, post-test CT scanning reveals the internal crack coalescence pattern and failure mode is impacted by the interactions of interbeds, natural fracture and pyrite bands. The rock bridge structure in marble with 60° and 90° interbed orientation alters the crack propagation path. Although the crack pattern is relatively simple at rock bridge segment, much more energy is needed to drive crack propagation. The testing results are expected to improve the understanding of the influence of rock structure on fracture evolution when subjected to variable stress amplitude loading conditions.

Rill erosion is a small-scale but universally occurring phenomenon. Given its potential to concentrate into larger-scale erosion and its non-negligible contributions to soil loss, substantial research has been dedicated to understanding its processes. In this article, we conducted a holistic review of the major achievements in rill erosion research over the past few decades, mainly from the following perspectives: 1) Hydraulic parameters to describe rill development; 2) morphological indicators to represent rill morphology; 3) commonly used measuring methods for rill morphology and rill flow; and 4) advantages and limitations of rill erosion modelling. In each of the perspectives, we also identified the challenges faced by current rill erosion research. Concrete suggestions on the pressing needs to help advance rill erosion research in the future are further presented.

The quantification of point source (PS) pollution and non-point source (NPS) pollution in different regions is needed so that targeted management strategies for water environment managers can be prepared. The aim of this study was to establish a method for quantifying PS and NPS pollution by measuring ion concentrations. The Danjiangkou Reservoir and its main tributaries were selected as the study area. The spatiotemporal variation in the concentration of nutrients and ions was analyzed. The results showed that the main ions did not display any distinct spatiotemporal variations on the reservoir, but spatiotemporal variations were evident in the tributaries: Mg2+, Na+, SO42− and Cl− were lowest in the wet season, K+ was highest in the dry-to-wet transition season, and Si was highest in the wet season. We selected Cl−/TN, SO42−/TN, and Na+/TN as PS pollution tracking indicators and K+/TN, Ca2+/TN, and Si/TN as NPS pollution tracking indicators. The mean contribution rate of NPS pollution calculated using Cl−/TN, Na+/TN, and Si/TN, and by the isotope approach, had a significant linear relationship (p

Soil redistribution is an important movement process of surface material that drastically affects the dynamic cycling of essential elements and soil productivity in the terrestrial ecosystem. Although a number of studies have focused on the soil organic carbon (SOC) pool under soil redistribution, few studies have comprehensively assessed the effects of soil redistribution on the ecosystem carbon pool. A five-year experiment of simulated topsoil redistribution (removal and addition, R-A) was conducted to explore the effects of soil redistribution on soil CO2 emissions, the SOC pool and the plant C pool in the semi-arid Loess Plateau of China. The soil properties, soil CO2 emissions, and crop parameters were measured after the topsoil removal (R20) treatment and topsoil addition (A20) treatment and in the undisturbed control (CK). Our results showed that 1) compared with the undisturbed control, the mean soil CO2 emissions of the R-A treatment decreased by 8% while the mean SOC pool increased by 3.0%; 2) the plant C pool was significantly reduced by 53.8% in 2015 and 5.8% in 2019 compared with the undisturbed control; 3) the ecosystem C pool (the sum of the SOC pool and plant C pool) of the R-A treatment was reduced by 10.3%, 3.7% and 1.1% compared with the ecosystem C pool of the undisturbed CK for 2015, 2016 and 2017, respectively, but increased by 0.6% and 1.9% from 2018 to 2019. During the first three experimental years, the decrease in the ecosystem C pool of the R-A treatment was caused by the reduced plant C pool in R20 exceeding the elevated plant C pool in A20. Over time, the soil bulk density (BD) decreased in R20 and the SOC, microbial biomass carbon (SMBC) and total nitrogen (TN) increased, which jointly improved the plant C pool in R20 and the ecosystem C pool by 2019. The gradual recovering pattern of the ecosystem C pool in the later years highlights the key role of plants in restoring soil properties and stabilizing ecosystem C cycling. Overall, our findings highlight that any attempt to assess the net impacts of soil redistribution, whether as a sink or source for atmospheric CO2, should comprehensively consider the changes in the SOC pool and plant C pool.

Slope induced runoff and erosion processes redistribute water, soil and nutrients, thus potentially influence soil CO2 emissions and perturb the carbon balances on sloping land. However, most of the previous studies on soil CO2 emissions were conducted on flat land. In regions with complex terrains and fragmented fields with terraces, such as the Chinese Loess Plateau, it is particularly important to understand slope dependent soil CO2 emissions. This requires investigations to unravel the relationship between soil erosional responses and CO2 emissions on different slopes. In this study, erosion plots, set at angles of 0.5°, 5°, 10°, and 20°, were refilled with loess soils from local farmland on the Chinese Loess Plateau. Changes of soil properties and CO2 emission rates were measured at three positions (upper, middle, lower) on each plot from October 2014 to September 2015. Our results show that greater runoff from the steeper slopes evidently depleted soil water and nutrients, directly responsible for the less soil CO2 emissions. Furthermore, the average yearly soil CO2 emissions decreased exponentially with slope gradients, from approximately 832.7 g m−2 yr−1 on the 0.5° slope, to 380.9 g m−2 yr−1 when the slope gradient was 20°. This not only proves the dependency of soil CO2 emission to slope gradients, but also provides the fundamental dataset to develop an empirical model between slope gradients and soil CO2 emissions. A coefficient β has been parameterized for the first time in this study to represent the sensitivity of soil CO2 emissions to slope gradients. The model of slope-dependent CO2 emissions identified in this study tentatively demonstrate non-negligible implications for global carbon fluxes from sloped and terraced field to the atmosphere. Further investigations on slope-scale carbon balances should test and integrate topography specific CO2 emissions.

Erosion-induced land degradation in the northeastern China, the Chinese breadbasket, has become one of the biggest risks to national food security. Apart from depleting topsoil, incising gullies, and waterlogging depositional sites, slope-scale erosion also spatially redistributes soil nutrients and introduces localized soil degradation. This requires to systematically understanding topography-specific spatial distribution patterns of eroded soil compositions and the relative abundance of each nutrient at different topographic settings. In this study, topsoil at predetermined space intervals were collected from two adjacent but differently curved segments (convex vs. concave) on a sloped cropland in northeastern China. The spatial distributions of topsoil soil organic carbon (SOC), total nitrogen (TN), total phosphorous (TP), and the variations of their atomic ratios were compared. Our results show that: (1) Compared with the nearby undisturbed tree rows, the SOC and TN was halved on the investigated slope, demonstrating great degradation since the land use conversion. (2) The spatial distribution of topsoil nutrients were evidently topography specific, with the SOC being comparable at slope summit, but declining from 27.57 to 4.72 g kg−1 (depletion ratio of 0.17) at the convex segment meanwhile accumulating up to 34.71 g kg−1 (enrichment ratio of 1.26) at the concave segment. Similar but more deviated spatial patterns were also observed on the TN and TP. (3) The atomic ratios of C:N:P also differed as the slope curvatures diverted, by being more variable from 51:3:1 to 105:6:1 along the convex segment but rather stable between 74:5:1 and 89:5:1 along the concave segment. (4) The lower content of SOC and the heavier δ13C along the convex segment suggest more advanced mineralization of C and thus greater susceptibility of C-limitation. Meanwhile, the slightly lower C:N ratios and the greater δ15N along the concave segment indicate greater stability of C, more advanced mineralization of N and thus likely a N-limited process. Overall, our findings demonstrate the controlling influences of minor geomorphic features to on-site soil dislocation and localized degradation at slope scale. Apart from general soil conservation measures, topography-specific practices must also be integrated to local land use policy to effectively target localized land degradation.

Irrigation with brackish water has been widely applied in many regions of the world as an efficient alternative to conserve freshwater resources. Although it has limited soluble salt content, brackish water can alter the physio-chemical properties and microbial communities of soil over time. In this study, a cotton field that utilized drip irrigation with brackish water for 10 years (Brackish field) was compared with a nearby field that utilized drip irrigation with fresh water (Fresh field). Our results show the following: 1) After 10 years, the Brackish field was saltier with a greater electrical conductivity and sodium adsorption ratio, as well as less soil organic carbon content when compared with the Fresh field. 2) The more polarized distribution of the most abundant operational taxonomic unit (OTU) was observed in the Brackish field, accompanied by significantly smaller beta diversity, a less clustered network and fewer co-occurrences (edges) within the soil microbial community compared with the less salty Fresh field, altogether demonstrating that long-term brackish irrigation was very likely to form a less diverse and more stressed environment for soil microbes to co-exist. 3) The mismatches between the edge ranking of hub species and the ranking of relative abundances highlight that the predominant abundances of salt tolerant species from genera Acinetobacter and Pseudomonas may help cotton to grow better in the saline soil but may not help to stabilize soil microbial communities. In future studies, sophisticated sampling protocols with high time-resolution are required to fully capture the possible soil microbial responses to event-sensitive ion redistributions across cotton rows as well during repeated irrigation cycles.

The availability and precision of topographic data determines the reliability of the calculated slope length and slope gradient (LS) factor, which limits the use of the universal soil loss equation (USLE) and its adapted versions in region-scale soil erosion risk assessment. One of the common complications is the effectiveness of topographic data with different resolutions or the compatibility of topographic data originated from different sources. In this study, the topographic data from five common sources, 5-m digital elevation model (DEM) based on 1:10,000 topographic maps (5-m topo DEM of 1:10,000), 25-m DEM derived from 1:50,000 topographic maps (25-m topo DEM of 1:50,000), 30-m ASTER GDEM, 30-m and 90-m SRTM DEMs, were individually applied to calculate the LS factors of five catchments with distinct terrain characteristics. Our results show that the computation accuracy of the LS factors in the five study catchments decreased with greater grid sizes derived from the five topographic data sources. Compared to the most precision 5-m topo DEM of 1:10,000, the relative computation error of the mean LS factors was less than 10% when calculated from the 25-m topo DEM of 1:50,000, and that was less than 25% from the 30-m ASTER and 30-m SRTM DEMs. For scenarios in gently rolling areas, the 90-m SRTM DEM with the relative computation error less than 15% could be recommended when there are noises on the open sources DEMs surface. Therefore, depending on the requirements of data accuracy, different data sources can be applied individually or in combined to obtain the optimal predictions of the LS factors. However, such recommendations on data sources proposed in this study appeared to be more applicable for regions with complex terrains. Further studies over a range of terrain features and spatial scales are required to validate the effectiveness of different topographic data sources in calculating the LS factor.

Soil redistribution by terrace construction, as one of the most evident anthropogenic imprints on hill-slopes, may influence soil organic carbon (SOC) dynamics through re-shaping topography and altering water and oxygen availability. However, the fundamental role and mechanisms by which terrace construction affects in situ soil CO2 emissions and its temperature sensitivity (Q10) remain poorly understood. In this study, topsoil removal-addition approach was used to simulate topsoil redistribution during terrace construction. Compared with the nearby undisturbed soil, the average annual soil CO2 emission over two years was reduced by 24% in the topsoil removed field but enhanced by 33% in the topsoil added field. The decreased soil CO2 emission at the topsoil removed field was largely associated with the depletion of SOC stocks and microbial biomass carbon, while the increments of SOC available for decomposition at the topsoil added field contributed to its increased soil CO2 emissions. However, the average Q10 value in the topsoil removed field was 23% greater at seasonal scale and 28% greater at diurnal scale than that in the undisturbed soil. The increased Q10 in the topsoil removed field is mainly due to higher aromaticity of water-extractable organic carbon (WEOC) and the domination of Actinobacteria in keystone taxa. Overall, our results show that changes in both aromaticity of WEOC and soil microbial community composition induced by soil redistribution during terrace construction may alter the response of soil CO2 emission to elevated temperature. Our study indicates that the impact of man-made soil redistribution should not be neglected when studying regional carbon cycling.

Climate change is expected to affect both the amount of global crop production, and annual variability in food supply. Agriculture is a major source of greenhouse gas emissions, but also considered to mitigate climate change. Conservation tillage, as a climate-smart agricultural practice, is repeatedly reported to mitigate net greenhouse gas emissions by increasing soil organic carbon (SOC). However, with reduced tillage, less litter is moved from the surface deeper into the soil profile, so SOC increase is very likely constrained to topsoil layers. Further adaptation benefits, such as increasing crop yield and resilience to famine, have recently been questioned after averaging yields from field studies. However, such global averaging masks the geographic extent individual studies apply to. This paper attempts a holistic regional analysis on the benefits of conservation tillage, in particular its fundamental principle no-tillage (NT), on the Chinese Loess Plateau. Based on a review of almost 20 years of conservation tillage plot experiments, the potential of NT to increase SOC stocks and to adapt to lower but more variable rainfall in the future has been assessed. The results show that the difference of total SOC stocks between NT and CT decreased with soil depth, confirming that the SOC benefits of NT are concentrated to the immediate topsoil still subject to direct seeding. The topsoil achieved maximum SOC stocks after about 10 years of NT. Crop yields from NT increased by up to 20% for years with average and below average precipitation, demonstrating the advantages of NT in stabilizing crop yields in dry years. However, the results in previous reports are not weighted by the actual spatial extent of drylands and humid regions after counting individual plot studies. As a consequence of such global and unweighted averaging, the benefits from NT to increase SOC stocks are likely to misrepresent the actual impact. Therefore, given the size of the Loess Plateau and its relevance for food security in China, our analysis illustrates the need to assess the benefits of a tillage and residue management system for each combination of eco-region and farming practice, weighted by their area and the affected population, rather than just using a global average for policy development on sustainable productivity.

Soil erosion and deposition occur widely from regional to global scales and have profound impacts on ecological services and sustainability. Despite their crucial roles in biogeochemical cycles, the responses of soil bacterial communities to soil erosion and deposition remain largely unclear. In this study, a field simulation experiment was conducted to examine variation in soil bacterial communities across eroding slopes and depositional zones with three slope gradients (5°, 10° and 20°) on the Loess Plateau of China (2015–2017). The results showed that soil physicochemical properties were altered by redistribution of runoff and sediment across eroding slopes and depositional zones. Soil bacterial alpha diversity was higher in the depositional zones of both 10° and 20° slopes compared with the 5° reference slopes but no markedly difference was found between eroding slopes and reference slopes. By contrast, bacterial community structure differed between eroding slopes and reference slopes but not between depositional zones and reference slopes. Differentiation of bacterial communities between eroding slopes and depositional zones increased with increasing slope gradients. The bacterial network was greater and more complex within depositional zone than eroding slope, indicating more extensive bacterial interactions and greater community stability potential. Erosion- and deposition-induced redistribution of soil moisture, soil organic matter, available P, and available K were the key determinants of variation in bacterial community structure. Our findings demonstrate the contrasting effects of soil erosion and deposition on soil bacterial communities, which should be given further attention across eroding landscapes.